I will be grateful to you if anybody explain me how can I do this.I am a newbie in after affects. I collect a after effect template from a site.I agree to customize it.I customized it but when I try to render them I can not select whole projects.I can only select one composition. please can you tell me how can I select whole project so that I can convert it to mp4 or .mov.

I can not render this template properly.In this case only select one composition

But I can render this project properly as it has a single composition.

I will be grateful to you if anybody explain me to- how can I converter first project(with multiple composition) to mp4 .

The other option is to select all your compositions in the project window and use the "add to render queue" command in the composition menu up top. You simply need to select all the comps in the render queue and choose your render settings, output format, and output location.